Neogene-Quaternary Tectonic Stratigraphy of the eastern Southern Alps, NE Italy

In order to reconstruct the Neogene-Quaternary Tectonic Stratigraphy of the eastern Southern Alps, the late Tortonian-lower Pleistocene foredeep clastic sequences, cropping out in the Veneto and Friuli piedmont areas have been extensively investigated focusing on the contractional features observed in the frequent conglomerate layers (pitted pebbles). The eastern Southern Alps is the result of a typical polyphase tectonic activity initiated in Late Oligocene, characterised by the occurrence of several non-coaxial stress regimes. Based on careful analyses of the pebbles' surfaces (shape and orientation of the indented features) and following a systematic and statistical approach, the mean orientation of the maximum compressive stress axis (σ1) has been obtained for more than 30 sites of measurements along the ca. 120 km-long investigated piedmont area. The affected lithostratigraphic units and the orientation (pre- versus post-tilting) of the stereonets density peaks make it possible to recognize four distinct deformational events and characterize them in terms of mean σ1 direction and timing: late Tortonian (σ1=313°/00°), late Messinian-Early Pliocene (338°/04°), Late Pliocene (314°/03°), and Early-Middle Pleistocene (160°/03°). The Tectonic Stratigraphy of the eastern Southern Alps during the last ca. 10 Ma shows the occurrence of several variations of the stress field characterized by a repeated oscillation of the σ1 axis between a NNW-SSE and NW-SE directions (Twist Tectonics). Taking also into account literature data about older deformational events (Chattian-Burdigalian and Serravallian-Tortonian), we analyse the Neogene-Quaternary tectonic evolution in the frame of the central Mediterranean realm and compare it with the convergence direction between the Adria-Africa and Europe lithospheric plates. We show that compressional directions within the eastern Southern Alps are basically governed by the remote plate convergence, though since late Messinian, the approaching Northern Apennines started to play a crucial role within the investigated area. Alternating short-lived phases (1-2 Ma) of coupling and decoupling along the basal detachment of the Apennines accretionary wedge probably caused temporary perturbations of the 'local' stress field and complex accommodation structures in this region of ongoing crustal collision.